This invention relates to the recovery of copper from copper ores.
In processes for the recovery of copper from copper-containing ores in which copper is first extracted from the ores using an acid leach solution, followed by contacting the acid leach solution with organic solvent solutions containing oxime extractants, problems have been found with the processing of ores from certain locations such as ores from some areas of Chile. In particular, unacceptable degradation of the oximes has been found to occur, resulting in very high levels of oxime requirements per ton of copper produced from the ore, which of course results in a serious economic disadvantage.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term xe2x80x9caboutxe2x80x9d.
It has been discovered that the copper ores that cause degradation of oxime extractants are those in which the ores contain concentrations of nitrate ions, which are taken up by the aqueous acid leach solutions. When organic water-immiscible solvent solutions containing oxime extractants come in contact with the nitrate-containing acid leach solution, especially those containing nitrate levels above 10 g/l, nitration and/or hydrolysis of the oximes has been found to take place, resulting in large losses of oxime extractants. In addition, buildup of the nitrated oximes, which become loaded with copper as a copper complex and which cannot be effectively stripped under commercial operating conditions, causes viscosity of the organic solvent solutions to increase to unacceptable levels, resulting in such problems as a lower net copper transfer to the organic phase, increased entrainment of the aqueous phase in the organic phase, increased entrainment of the organic phase in the aqueous phase, and precipitation of the nitrated oxime copper complex from the organic phase. In some instances, the above problems have resulted in oxime extractant consumption of about eight times the oxime consumption when aqueous acid leach solutions which do not contain nitrate ions are processed in an otherwise identical manner.
In investigating the above problems it was discovered that
a) the presence of oxime extractant modifiers and/or additives significantly increased the rate of oxime degradation;
b) the presence of large quantities of aldoximes in the oxime extractants significantly increased the rate of oxime degradation;
c) when the acidity of the aqueous phase in contact with the organic phase was reduced to a pH range of from 2.25 to 3.1 oxime degradation was significantly reduced; and
d) an electromotive force (EMF) of 650 mV or larger, as measured against an Ag/AgCl electrode, in the aqueous phase in contact with the oxime-containing organic solvent solution significantly increased oxime degradation.
e) other relationships between pH, EMF, and nitrate levels, described hereinafter.
Accordingly, the present invention relates to the following process variants, used individually or in combination, for reducing oxime extractant degradation from contact with the nitrate ion-containing aqueous phase in contact with the organic phase;
A) use of oxime extractants in water-immiscible organic solvent solutions wherein the solutions do not contain any modifiers or kinetic additives for the oxime extractants;
B) use of oxime extractants containing only ketoximes or a mixture of ketoximes and aldoximes in which the ketoxime:aldoxime molar ratio is less than 1:1.2, preferably less than 1:0.5, and more preferably 1:0.25 or less;
C) increasing the pH of the aqueous phase to a pH in the range of from 2.25 to 3.1 prior to contact with the oxime-containing organic solvent solution;
D) reducing the electromotive force in the aqueous phase to less than 650 mV, preferably to less than 600 mV, before contact with the organic solvent oxime extractant solution (organic phase);
E) when the pH of the aqueous phase is at or above 0.95, and the NO3xe2x88x92 level is 32 g/l or less, the EMF can be above 650 mV, e.g. as high as 700 mV. However, when the EMF is greater than 700 mV, e.g.  greater than 700-800 mV, then (a) reduce the EMF to 700 mV or less, and/or (b) increase the pH to greater than 1.2, and/or (c) decrease the NO3xe2x88x92 level to 25 g/l or less prior to contact with the organic phase;
F) when the pH of the aqueous phase is lower than 0.95, e.g. from 0.5 to  less than 0.95, then (a) the EMF must be, or be reduced to, less than 650 mV and/or (b) the NO3xe2x88x92 level must be, or be reduced to, less than 25 g/l, and/or increase the pH to 0.95 or greater prior to contact with the organic phase;
G) when the NO3xe2x88x92 level in the aqueous phase is greater than 32 g/l, e.g.  greater than 32-40 g/l or more, then (a) the pH is increased to 1.2 or more, and/or (b) the EMF is decreased to less than 650 mV, and/or (c) the NO3xe2x88x92 level is reduced to 25 g/l or less prior to contact with the organic phase;
H) when the NO3xe2x88x92 levels in the aqueous phase are, or are reduced to, 20 g/l or less, preferably 15 g/l or less, and more preferably 10 g/l or less, it is not necessary to control the pH or the EMF.
In the practice of the invention all EMF values and measurements are based on the Ag/AgCl electrode.
The solvent extraction process for extracting copper from copper ores typically involves the following steps:
1. Aqueous acid leaching of the copper ore using a strong acid to form an aqueous acid leach solution containing copper ions and often relatively small quantities of other metal ions. The aqueous leach acid solution dissolves salts of copper and other metals if present as it is contacted with the ore, e.g. as it trickles through the ore. The metal values are usually leached with aqueous sulfuric acid, producing a leach solution having a pH of 0.9 to 2.0.
2. The copper-pregnant aqueous acid leach solution is mixed in tanks with an oxime extraction reagent which is dissolved in a water-immiscible organic solvent, e.g., a kerosene or other hydrocarbons. The reagent includes the oxime extractant which selectively forms a metal-extractant complex with the copper ions in preference to ions of other metals. The step of forming the complex is called the extraction or loading stage of the solvent extraction process.
3. The outlet of the mixer tanks continuously feeds to a large settling tank or equivalent equipment, where the organic solvent (organic phase), now containing the copper-extractant complex in solution, is separated from the partially depleted aqueous acid leach solution (aqueous phase). This part of the process is called phase separation. Usually, the process of extraction is repeated through two or more mixer/settler stages, in order to more completely extract the copper.
4. After extraction, the partially depleted aqueous acid leach solution (raffinate) is either recycled for future leaching, or recycled with a bleed, or discharged.
5. The loaded organic phase containing the dissolved copper-extractant complex is fed to another set of mixer tanks, where it is mixed with an aqueous strip solution of concentrated sulfuric acid. The highly acid strip solution breaks apart the copper-extractant complex and permits the purified copper to pass and concentrate in the strip aqueous phase. This process of breaking the copper-extractant complex is called the stripping stage, and the stripping operation is repeated through two or more mixer-settler stages to more completely strip the copper from the organic phase.
6. As in the extraction process described above (step 2 and 3), the copper pregnant aqueous acid leach solution is fed to another settler tank for phase separation, or to another type of solvent extraction equipment that replaces the traditional mixer-settler.
7. From the stripping settler tank, the regenerated stripped organic phase is recycled to the extraction mixers to begin extraction again, and the copper is recovered from the strip aqueous phase, customarily by feeding the strip aqueous phase to an electrowinning tankhouse, where the copper metal values are deposited on plates by a process of electrodeposition.
8. After obtaining the copper values from the aqueous solution, the solution, known as spent electrolyte, is returned to the stripping mixers to begin stripping again.
The oxime extractants used in the above process are oxime extractants of the hydroxy aryl ketone oxime type or a mixture thereof with hydroxy aryl aldoximes. A general formula for such oximes is given on formula I shown below: 
in which A can be: 
where R and Rxe2x80x2 can be individually alike or different and are saturated aliphatic groups of 1-25 carbon atoms, ethylenically unsaturated aliphatic groups of 3-25 carbon atoms or ORxe2x80x3 where Rxe2x80x3 is a saturated or ethylenically unsaturated aliphatic group as defined; n is 0 or 1; a and b are each 0, 1, 2, 3, or 4, with the proviso that both are not 0 and the total number of carbon atoms in Ra and Rxe2x80x2b is from 3 to 25, Rxe2x80x2xe2x80x3 is a saturated aliphatic group of 1-25 carbon atoms or an ethylenically unsaturated aliphatic group of 3 to 25 carbon atoms, with the proviso that the total number of carbon atoms in Ra and Rxe2x80x2xe2x80x3 is from 3-25. Preferred compounds where A is (i) above are those in which a is 1, b is 0, R is a straight or branched chain alkyl group having from 7 to 12 carbon atoms and where R is attached in a position para to the hydroxyl group. Among those, the more preferred compounds are those wherein Rxe2x80x2xe2x80x3 is methyl and R and a are as designated. Compounds wherein n has a value of 0 (i.e. hydroxybenzophenone oxime compounds) can be prepared according to methods disclosed in Swanson U.S. Pat. Nos. 3,952,775 and 3,428,449. By reason of ready solubility in organic diluents commonly employed in solvent extraction and desirable properties of complexes of the compounds with copper, preferred benzophenone compounds are those having a single alkyl group of 7-12 carbon atoms in a position para to the hydroxyl group, in which the alkyl group is a mixture of isomers. Examples of such compounds are 2-hydroxy-5-nonylbenzophenone oxime and 2-hydroxy-5-dodecylbenzophenone oxime, which are obtained as mixtures of the isomeric forms when commercial nonylphenol and dodecylphenol are respectively employed in their synthesis.
Compounds wherein n has a value of 1 (i.e. hydroxy phenyl benzyl ketone oxime compounds) can be prepared according to methods described in Anderson U.S. Pat. No. 4,029,704. Preferred phenyl benzyl ketone oximes like the above noted benzophenone oximes are those having an isomeric mixture of 7 to 12 carbon alkyl groups as a single substituent on the ring para to the hydroxyl group. These preferred compounds are exemplified by the compound, 2-hydroxy-5-nonylphenyl benzyl ketone oxime, as manufactured from a commercial nonylphenol comprising a mixture of nonyl isomeric forms.
Compounds of the hydroxy phenyl alkyl ketone oxime type can be prepared according to the procedures disclosed in UK Patent 1,322,532, and are especially preferred for use herein. As noted with regard to the benzophenone and phenyl benzyl ketone compounds described above, the preferred compounds of this type are also those having an isomeric mixture of 7 to 12 carbon alkyl groups as a single substituent on the ring para to the hydroxyl group. Also preferred are those in which the Rxe2x80x2xe2x80x3 alkyl group is methyl. Illustrative of such preferred compounds where A is CH3 is 2-hydroxy-5-nonylphenyl methyl ketone oxime manufactured through the use of commercial nonylphenol.
Hydroxy aryl aldoxime extractants which can be employed in mixtures with ketoximes are those in which A is H. These hydroxy benzaldoximes, (also called xe2x80x9csalicylaldoximesxe2x80x9d), can be prepared according to methods described in Ackerley et al. U.S. Pat. No. 4,020,105 or U.S. Pat. No. 4,020,106 or by oximation of aldehydes prepared according to Beswick U.S. Pat. No. 4,085,146. Again preferred compounds are those having an isomeric mixture of isomeric 7 to 15 carbon alkyl groups as a single substituent para to the hydroxyl group, mixed alkyl isomeric forms of 2-hydroxy-5-heptyl benzaldoxime, 2-hydroxy-5-octyl benzaldoxime, 2-hydroxy-nonyl benzaldoxime and 2-hydroxy-5-dodecyl benzaldoxime are preferred, the most preferred for the purpose of the present invention where A is H is the dodecyl compound, i.e. 2-hydroxy-5-dodecyl benzaldoxime.
In one embodiment of the process of the present invention, the oxime extractant is either one or more ketoximes of formula (I) (i) or (I) (ii) or a mixture of one or more such ketoximes with one or more aldoximes of formula (I) (iii) above, in which the ketoxime:aldoxime molar ratio is less than 1:1.2, preferably less than 1:0.5, e.g. from 1:0.49 to 1:0.05, and more preferably is 1:0.25 or less.
The oxime extractants in the above process are typically used in prior processes in conjunction with modifiers such as one or more equilibrium modifiers, and kinetic active substances. Equilibrium modifiers include long chain aliphatic alcohols such an n-hexanol, 2-ethylhexanol, isodecanol, dodecanol, tridecanol, hexadecanol, and octadecanol; long chain alkylphenols such as heptylphenol, octylphenol, nonylphenol and dodecylphenol; organophosphorus compounds such as triloweralkyl (C4 to C8) phosphates, especially, tributyl phosphate and tri(2-ethylhexyl)phosphate; and either saturated or unsaturated aliphatic or aromatic-aliphatic esters containing from 10 to 30 carbon atoms, ketones, nitrates, ethers, amides, carbamates, carbonates, and the like. Kinetic active substances include xcex1,xcex2-hydroxy oximes described in Swanson, U.S. Pat. No. 3,224,873 and xcex1,xcex2-dioximes described in Koenders et al., U.S. Pat. No. 4,173,616.
In another embodiment of the invention, equilibrium modifiers and kinetic active substances are not used in the practice of the present invention, since they have been found to markedly increase the rate of oxime degradation from the nitrate ions.
In a further embodiment of the invention, it has been discovered that the addition of sodium sulfate (Na2SO4) to the copper-pregnant aqueous acid leach solution provides a buffering effect to increase the pH of the resulting raffinate obtained from the extraction of the copper-pregnant aqueous acid leach solution with the oxime extractant reagent dissolved in a water-immerscible organic solvent. The sodium sulfate is preferably added to the acid leach solution in a buffering-effective quantity, which is usually in the range of from 10 to 100 gpl, preferably from 10 to 50 gpl.
The water-immiscible organic solvents used in the solvent extraction process of the invention are usually water-immiscible liquid hydrocarbon solvents. These include aliphatic and aromatic hydrocarbon diluents such as kerosene, benzene, toluene, xylene and the like. A choice of essentially water-immiscible hydrocarbon solvents or mixtures thereof will depend on factors, including the plant design of the solvent extraction plant, (mixer-settler units, extractors) and the like. The preferred solvents for use in the present invention are the aliphatic or aromatic hydrocarbons having flash points of 130xc2x0 Fahrenheit and higher, preferably at least 150xc2x0 and solubilities in water of less than 0.1% by weight. The solvents are essentially chemically inert. Representative commercially available solvents are CHEVRON(copyright) ion exchange solvent (available from Standard Oil of California) having a flash point of 195xc2x0 Fahrenheit; ESCAID(copyright) 100 and 110 (available from Exxon-Europe) having a flash point of 180xc2x0 Fahrenheit; NORPAR(copyright) 12 (available from Exxon-USA) with a flash point of 170xc2x0 Fahrenheit; CONOCO(copyright) C1214 (available from Conoco) with a flash point of 160xc2x0 Fahrenheit and C 170 exempt solvent having a flash point above 150xc2x0 Fahrenheit; and Aromatic 150 (an aromatic kerosene available from Exxon-USA having a flash point of 150xc2x0 Fahrenheit), and other various kerosene and petroleum fractions available from other oil companies, such as the ORFOM(copyright) SX series of solvent extraction diluents (available from Phillips 66 Company: SX 1, 7, 11 and 12 each having a flash point above 150xc2x0 Fahrenheit varying up to 215xc2x0 Fahrenheit); and ESCAID(copyright) series of hydrocarbon diluents (available from Exxon: 100, 110, 115, 120, 200 and 300, each having a flash point above 150xc2x0 Fahrenheit; and EXXOL(copyright) D80 solvent (also available from Exxon and having a flash point above 150xc2x0 Fahrenheit).
In the process, the volume ratios of organic to aqueous (O:A) phase will vary widely since the contacting of any quantity of the oxime organic solution with the copper containing aqueous solution will result in the extraction of copper values into the organic phase. For commercial practicality however, the organic (O) to aqueous (A) phase ratios for extraction are preferably in the range of about 50:1 to 1:50. It is desirable to maintain an effective O:A ratio of about 1:1 in the mixer unit by recycle of one of the streams. In the stripping step, the organic:aqueous stripping medium phase will preferably be in the range of about 1:4 to 20:1. For practical purposes, the extracting and stripping are normally conducted at or close to ambient temperatures and pressure although higher and lower temperatures and pressures are entirely operable although higher temperatures will increase oxime degradation. While the entire operation can be carried out as a batch operation, most advantageously the process is carried out continuously with the various streams or solutions being recycled to the various operations in the process for recovery of the copper metal, including the leaching, extraction and the stripping steps.
In the extraction process, the organic solvent solutions can contain the oxime extractant typically in an amount of about 2 to 35 weight/ volume %.
After stripping of the copper values from the organic phase by the aqueous stripping solution and separation of the organic and aqueous stripping phase, the copper metal can be recovered by conventional recovery processes, including, but not limited to, precipitation and electrowinning. Electrowinning is typically the preferred means of recovery of the copper from solutions suitable for electrowinning, generally highly acidic aqueous solutions, such as a sulfuric acid solution containing about 5 to about 200 g/l sulfuric acid, which is preferred as the aqueous acidic stripping solution to remove the copper values from the organic phase.
It is at this step, the stripping step, that the extraction reagent organic circuit phase which has degraded is removed from the extraction circuit after stripping for reoximation, prior to recovery of the metal from the stripping solution, with an optional scrub or wash step to remove any residual metals from the organic phase prior to reoximation and optional purification distillation where necessary or desirable.
Also, prior to stripping, it is not unusual to wash the organic phase, particularly where trace metals may be loaded on the organic extractant and/or aqueous phase is entrained in the organic phase. One or more wash stages may accordingly be employed depending on any trace metals present, the amount of entrainment and the required purity of the final copper loaded stripping solution.
In a further embodiment of the invention (variant C)), the acidity of the aqueous phase is reduced to a pH in the range of from 2.25 to 3.1, preferably from 2.25 to 2.7 prior to contact with the oxime-containing organic solvent solution. The acidity reduction can conveniently be carried out by adding a base to the aqueous phase, in which the base does not form an insoluble compound with the copper ions in the leach solution at a pH of 2.25 to 3.1. Sodium hydroxide is a preferred base for use herein. Other bases that can be used include sodium sulfate. Alternatively, acidity reduction can be carried out by contacting the aqueous phase with an acid-consuming oxidic copper ore.
In another embodiment of the process of the invention (variant D)), the electromotive force of the aqueous phase is reduced to less than 650 mV, preferably less than 600 mV, as measured against an Ag/AgCl electrode, before contact with the oxime extractant-containing water-immiscible organic solvent solution in step 2. of the process. Various techniques can be employed to reduce the electromotive force. For example, the above aqueous phase can be contacted with a reducing agent such as a metal that is more active in the electromotive series than copper, such as iron, aluminum or magnesium, which will not result in the introduction of new ions into the aqueous phase. Other more active metals such as zinc can also be employed, but such metals are less desirable since they are usually not otherwise present in the aqueous phase, and may introduce unwanted effects on the process. In addition to the above metals, copper metal can also be used as a reducing agent. When copper or iron metal is used, they are preferably in the form of scrap metal or other divided forms of these metals. Contact is preferably made by passing the aqueous phase over the selected metal reducing agent prior to step 2 of the process.
Reducing agents other than active metals can also be employed. For example, compounds such as sulfur dioxide, and alkali metal and alkaline earth metal sulfites or bisulfites can be employed. Such reducing agents result in the formation of sulfurous acid when added to the aqueous phase. The most preferred reducing agents are sulfur dioxide, sodium sulfite, and sodium bisulfite. Ferrous sulfate, either in solid form or in aqueous solution, can be added to the aqueous phase to both reduce and stabilize the EMF.
This embodiment of the invention can also be carried out by removing a portion of the aqueous phase, preferably from 2.5% to 50% by weight more preferably from 2.5% to 30% by weight, and most preferably from 5% to 25% by weight, and treating only this portion of the aqueous phase with a reducing agent, and then mixing the treated portion with the untreated portion of the aqueous phase. It has been discovered that this procedure will reduce the EMF to the desired level, i.e., less than 650 mV, and typically less than 600 mV. This variation results in significant potential savings in capital since equipment size can be significantly reduced. Aqueous fee solution flows are typically very large. If the aqueous feed solution flow is on the order of 1000 m3/hr, for example, it is only necessary to treat a portion of it, e.g., 10% of 1000 m3/hr. Depending on the nature of the aqueous solution, the amount of solution that must be treated to produce a given EMF that will give the desired EMF when mixed back with the incoming feed solution can be readily determined by those skilled in this art.
More than one reducing agent can be employed, preferably by taking a portion of the aqueous phase, treating it with an SO2 producing reducing agent, contacting the resulting aqueous phase with copper or iron scrap metal for a period of about 15, e.g. 5 to 30 minutes, and then returning this portion of the aqueous phase to the remainder of the aqueous phase. The quantity of SO2 added to the aqueous phase (based on the total quantity of aqueous phase) is not critical, and is generally in the range of 0.1 g. to 2 g. of SO2 per liter of leach solution.
In further embodiments of the invention E) through H) when the NO3xe2x88x92 levels are reduced, this reduction can be accomplished by either (i), extracting the aqueous phase with a base, such as a tertiary alkylamine, e.g. ALAMINE(copyright) 336 (tri(C8-C10) amine), ALAMINE(copyright) 304-1 (trilaurylamine), ALAMINE(copyright) 308 (tri-isooctyl amine), ALAMINE(copyright) 310 (tridecyl amine), ALAMINE(copyright) 300 (tri-n-octyl amine), and the like; (ii) removing the nitrate by treatment of the aqueous phase with a strong sulfuric acid solution, e.g. 200-400 g/l sulfuric acid, to convert the nitrate to nitric acid, and volatilizing the nitric acid by heating, e.g. 50xc2x0 C. to 120xc2x0 C.; or (iii) selectively removing the nitrate ions by passing the aqueous phase over a semi-permeable membrane such as that used in the so-called HW process.
When the variants of the invention involve increasing the pH of the aqueous phase, this can be accomplished by treating the aqueous phase in accordance with the methods described above for variant C).
The invention will be illustrated but not limited by the following examples.